Ziggy:
I'd check on
http://www.acme.com/planimeter/ to see if the pond is on there. You can get an accurate measurement that way.
As for the FA, there has to be some high nutrient loading for the pond to grow that much that quickly. Is the stream water high in nutrients? The well water should not be. Run-off from a feed lot, pasture, etc.? If the pond is that large, then the numbers of fish shouldn't be a problem. How much food are you going thru in a month?
Manual removal would be the best, although it's a PITA. Raking it out and removing it from the pond basin would be the best. If you just kill it the nutrients are still in the pond and the cycle continues.
I wonder if a semi-trash pump would work to suck up the algae, spit it up on the bank, but allow the water to run back into the pond without carrying the algae back in with it? Maybe spit the water on the opposite side of a silt fence??